Grammar by osmosis
Even after speaking their first words, children continue to learn language in a very predictable sequence that is remarkably similar throughout the world (also among deaf children), even as the individual differences between children become more pronounced. Shortly after the acceleration in vocabulary, the grammar is ready for take-off. Grammatical learning is closely related to the critical period of language learning. Just as in the first year of life, the child is extremely sensitive to sounds leading to flawless pronunciation; the third year is key to a native command of the grammar. Vocabulary can always be enhanced later in life, but pronunciation and grammar remain the dead giveaways of a non-native speaker, as both are next to impossible to perfect as an adult.
In a study at MIT, researchers had 16-18 month olds watching two simultaneous clips of Sesame Street. One of the sequences showed "Cookie Monster is Tickling Big Bird" on one screen and "Big Bird is Tickling Cookie Monster" on the other. A voice-over played one of these sentences, and the babies consistently looked more at the video corresponding to the voice. The importance of this first grammatical step is moving from the single word stage to two-word sentences. "Mommy drive." "Nice doggy." The two-word stage typically sets in around the age of 2 - 2.5 years. Children remain a while on this plateau, just as they do with their first few words.
Although every language has grammatical rules, a great deal must be learned by heart -- not just the usual verb conjugations, but also the ever tricky prepositions and other subtleties. For example, the Dutch always sit 'behind' the computer and are 'under' the shower. The daunting task of internalizing all of these grammatical rules and their exceptions is what the brain of a 3-year old does without flinching.
Surprisingly enough, there are only two basic ways to change meaning for all languages: Change of word order or the little (annoying) bits tacked on to the beginning or end of words prefixes and suffixes. Babies understand this well before they can speak themselves.
Adding the finer points
There is actually no three-word stage. The baby goes from two words directly into simple sentences. These first sentences develop as combinations of an ever-increasing number of words together. They may be correct in word order, but lacking some finer points, such as inflections and little function words. "Baby pat puppy dog." "Want go home now." "Where go mommy car?" This stage is, for obvious reasons, called 'telegraphic speech.' The path from here to properly formed sentences is again strikingly predictable. English-speaking children typically begin with present tenses, present participle verb endings (-ing form) in particular: "Where Daddy driving? " Next come the prepositions (in, on, at, out), adjectives (small, yellow, hot), followed by plural endings (-s) like in horses; possessive endings (-s) like in his; articles (a, the), adverbs (fast, slowly, carefully), regular past tense endings (ed) as in jumped; pronouns (me, you, us, them); and third person present tense endings (-s) as in "the bear sleeps."
Incredibly, babies don't just know the rules, they truly understand their logic. When presented with an invented word during an experiment at MIT and shown a drawing of a fantasy creature -- say, a wug -- and then shown a second wug, the English-speaking baby invariably said "wugs." Even more interesting are the mistakes they make. "We swimmed" is a perfect example of baby applying a perfectly logical rule to a language that has a number of irregularities and exceptions. This 'over-generalization' persists until the child has sorted out the instances where the general rules apply and those where they have to be overruled by the irregular verb.
Bear in mind that correcting your child doesn't help at all. Before the age of three, children simply don't understand language corrections and can instead become self-conscious, ashamed, and quiet. It's best to respond to "We swimmed," with "Yes, you swam in the pool today! I swam too. You swam really well!" You will achieve much better results by offering modeling and repetition, instead of correcting. Ideally, see if you can make him repeat the word later. ("Can you tell Sara what we did today?").
By the time he turns three, your child will be a pretty sophisticated talker. He'll be able to carry on a sustained conversation adjusting his tone, speech patterns, and vocabulary to his conversation partner. For instance, he'll use simpler words with a peer, but be more verbal with you. By now, he may be almost completely intelligible in both languages. He'll even be a pro at saying his name and age and will readily oblige when asked.
Look at the different sections for what baby understands and can say at these stages:
- 0-12 months: The first year starts small, and is all about sounds. The early language milestones are virtually identical for all babies, regardless of the language or number of languages learned.
- 12-24 months: The second year is all about words and linking words to objects. Just as with monolinguals, this is a highly individual phase. Be patient, and you will be richly rewarded with the first words, in one or several languages.